1. Field of the Invention
The present invention is an improvement on the arrangements for the application of solid particles on the image carrier of a non-impact printer such as described in U.S. Pat. Nos. 4,246,588 issued on Jan. 20, 1981, and 4,348,684 issued on Sept. 7, 1982 to Compagnie Internationale pour l'Informatique CII-HONEYWELL BULL.
2. Description of the Prior Art
The present invention finds particular application to known forms of printing devices, such as non-impact or strike-less printers, in which the printing of characters is achieved without recourse to raised type impacting on a recipient sheet of paper.
Printing machines of this sort generally comprise an image carrier, most often constituted by a rotary drum or an endless belt on the surface of which sensitized zones can be formed by electrostatic or magnetic means. These zones, also referred to as latent images, correspond to the characters or figures to be printed. These latent images are then developed, i.e. made visible by means of a powdered developing pigment which, deposited on the image carrier, is attracted only by the latter's sensitized zones. The pigment particles which have thus been deposited on these latent images are thereafter transferred to a support sheet, such as a sheet of paper, to which they are affixed in a permanent manner.
To apply this powdered developing pigment to the image carrier of a printing machine of this type, various application arrangements have been used in the prior art. One form of arrangement which has been used includes a cylindrical housing containing the powdered pigment. This housing is provided with an aperture in front of which passes the image carrier. The inking of this carrier is effected by a cylindrical brush, rotating inside the casing and which projects the pigment particles toward the surface of the carrier as the carrier passes before the aperture in the housing.
This arrangement is not entirely satisfactory in practice, however, owing to the fact that it causes a cloud of pigment particles to be formed that spreads outside the housing. This is specially unpleasant for persons who come into contact with this cloud when standing near the printer. Another undesirable feature is that the particles become electrified in an undesirable way which, after being thrown onto the carrier, may cause them to cling to the unsensitized zones by electrostatic attraction.
The arrangements described in the aforenoted patents remedy these drawbacks. For example U.S. Pat. No. 4,246,588 discloses an arrangement for applying to the image carrier of a non-impact printer solid particles contained in a tank comprising a conveying member designed to carry these particles to the vicinity of the image carrier, said arrangement being characterized in that it also includes a deflector inserted between the carrier and the conveying member to collect the particles conveyed, the deflector has one of its edges disposed within the immediate proximity of said carrier so as to form with this carrier a trough of essentially prismatic shape in which are accumulated the particles thus collected, the carrier is moved in the direction in which it entrains these particles toward the edge of said prism. The particles entrained beyond this edge remain attached only to the sensitized zones of said carrier.
But the arrangement described in U.S. Pat. No. 4,246,588 presents the drawback of requiring frequent refilling of the tank containing the solid particles. The reason is that the conveying member, consisting for example of a rotary cylinder, ceases to supply particles to the image carrier as soon as the particle level in the tank falls below the bottom of the conveying member.
To remedy this drawback it is possible to imagine placing a first worm screw above the conveying member in order to supply it with the solid particles, as well as a second worm screw positioned parallel to the first one and with opposing thread so as to compensate for the lateral movement of the particles which would be caused by the first worm screw if it were to operate by itself. This prevents a total draining of the tank, owing to the stirring of the particles resulting from the movement of the two screws, and it is no longer necessary to fill the tank frequently.
However, worm screws are expensive pieces of equipment. Furthermore, they cannot be fully immersed in the powdered pigment because, on the one hand, the mechanical power then needed to turn these screws becomes very great and, on the other hand, in the case where the powdered pigment is made of particles coated with a thermoplastic resin, this pigment is subjected to milling and lamination by the screws, causing the resin to melt owing to the resulting friction, and to produce particle packing. In addition, some particles are pushed back against the various bearings provided in the tank walls to ensure rotation of the screws, and binding of the corresponding axes of rotation will result since there are no simple and effective means to seal the bearings. Also, the screws feed the particles to the conveying member in intermittent fashion, which creates concentrations of particles on the conveying member and, conversely, zones that are deprived of these particles, which is detrimental to the application of said particles to the conveying member.